In a wireless communication system, a mobile station may access a wireless communication network using a radio link to a radio access unit of the wireless communication network. The radio access unit can be for instance a base station of a mobile communication network.
Occasionally, a mobile station which is connected to a specific radio access unit has to be handed over to another radio access unit of the wireless communication network. For instance, when the mobile station is moving, it may leave the coverage area of the radio access unit to which it is currently connected.
In many wireless communication systems, the radio access unit to which a mobile station is handed over has to be provided with information about the mobile station, in order to be able to guarantee a smooth transition of the connection. Such information may comprise in particular an activity state of the mobile station.
A possible activity state may be for instance an Active state, in which the mobile station continuously receives relevant control information provided by the wireless communication network, for example in every frame. Another possible activity state may be for instance a Sleep state, in which the mobile station occasionally receives information on whether it should activate itself. In a 3GPP mobile communication system, the mobile station typically enters the Sleep state upon a command issued by an L2 entity in the base station to which it is associated. The base station usually issues this command when a predetermined time has elapsed after the last communication of user data by the mobile station.
Various other activity states are also possible. The technical specification 3GPP TS 25.331 V6.1.0 (2004-03) “Radio Resource Control (RRC)”, Release 6, for example, gives an overview of RRC states within a Universal Terrestrial Radio Access (UTRA) RRC connected mode, including a URA_PCH state, a CELL_PCH state, a CELL_FACH state and a CELL_DCH state.
The type and the time of entry of the state of a mobile station which was entered before a handover might have to be known to a radio access unit to which the mobile station is connected after the handover. Moreover, the entry of a new state of a mobile station, which should be initiated by the radio access unit to which the mobile station is currently connected, may depend on an event which occurred while the mobile station was still connected to another radio access unit.
In current mobile communication networks, for example in networks defined by 3GPP and 3GPP2, information on the activity state of a mobile station is transferred within the network from the source base station to the target base station, possibly via intermediate nodes. This operation is typically referred to as a context transfer.
A conventional context transfer in a Universal Mobile Telecommunication Services (UMTS) network is illustrated in the diagram of FIG. 1.
In FIG. 1, a first vertical line on the left hand side represents a mobile station (MS). A second vertical line in the middle represents a first radio network controller RNC1 of the UMTS network, to which a first base station is associated. A communication between the first radio network controller RNC1 and the mobile station can be performed via this first base station. A third vertical line on the right hand side represents a second radio network controller RNC2 of the UMTS network, to which a second base station is associated. A communication between the second radio network controller and the mobile station can be performed via this second base station. The activity state of a mobile station accessing the UMTS network is kept in the radio network controllers. If the radio network controllers belong to the same radio access network, they may be directly connected to each other. Otherwise, they may communicate with each other via a core network to which they are connected.
The mobile station currently accesses the mobile communication network via the first base station. When a handover to the second base station is to be performed, the first radio network controller transmits to the mobile station a command to attach to the new cell. The mobile station transmits an acknowledgement ‘ACK’ of this command to the first radio network controller.
Upon receipt of the acknowledgement, the first radio network controller performs a network internal context transfer to the second radio network controller. In this context transfer, the activity state of the mobile station is indicated to the second radio network controller.
Moreover, the mobile station transmits a message to the second radio network controller, in order to make its presence known in the new cell covered by the second base station.
The context transfer from the first radio network controller to the second radio network controller via the mobile communication network has the disadvantage that it increases the latency of a handover. This is of particular relevance in 4G (fourth generation) mobile communication systems, in which the cell size of the mobile communication network is relatively small, which leads to frequent handovers when a mobile station is moving.